1992 Ford F350 intermittant fast idle problem...

ord.sgt.26NC

ord.sgt.26NC

Gott mit uns!
Joined
Mar 23, 2005
Location
Goldsboro
ok,specs:92 f350,351,zf 5 speed,192,000 miles.

problem description:I can start the truck up. Sometimes in a few minutes,the RPM's will increase a bunch and stay at that rpm.It might drop back down,or it might not.It might rise and fall like it's unsure where it wants to be.

It happens more now that it is hot than back during the winter.Then again I might can drive from here to UNF and it not act up at all or I might not make it to the main road here at the house before it does it.

When it acts up,I unplug the IAC motor and it drops down and I can drive it but it does have a rough idle.

I have replaced the IAC motor 3 times,cleaned the throttle body,sprayed for vacuum leaks,wiggled wires etc.

I am now wondering if the ECT sensor is sending screwy stuff to the EEC IV computer .

Any ideas from folks who know and have run across this problem? I'm not looking for the typical " replace such and such sensor,it might fix it" answeres. I can switch out sensors all day long.What I guess I am looking for is an answere from somebody who has seen this and what they did fixed the problem.
 
More than likely you have a bad TPS. If you have a DVOM you can check the output voltage. One wire will be ground, one will be 5 volt reference from the PCM and the other will be the signal wire, this is the wire you want to watch the voltage. Check this with the key on engine off. With the throttle closed you should have no more than 1 volt on the signal wire then watch the voltage increase to about 4.8 volts at WOT. The easiest way to replace the TPS is to remove the throttle body. It can be done without removing it but it's a bit tough.
 
yea,it's on the bottom of the throttle body with all kinds of hoses and vacume lines around it. I also noticed that when it revs up,the throttle plate doesen't move.

You are saying then is the TPS might be sending bad info to the PCM and then the PCM is compensating by signalling the IAC to increase RPM's?
 
x2 on the tps. I worked on a 1996 f350 a while back with a 351 that would idle at 3000 rpms all of the time. Replaced tps. For some reason I had to get one from the ford place. The aftermarket ones did not have the right connection. FORD # E7TZ9B989D
 
I wish whatever the problem was would do it all the time instead of intermittantly.It's just agravating more than anything else.Is the TPS a plug and play replacement or is there an adjustment sequence you have to follow?
 
ok,snatched the throttle body off and cleaned it up.Changed the TPS and ECTS while I was at it.Keep your fingers crossed.Ran her in the yard for over 30 minutes while I was working on my Willys and it idled fine.I'm gonna change the plugs now and then later on drive her to a M and G and see how she does.Thanks for all the help.Update later.
 
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